Electrical power is required for operation of a wide variety of devices that provide convenience or answer necessities in many aspects of modern life and human activities. At the present time, electrical power is distributed from locations where it can be conveniently generated to locations where it is utilized as alternating current or voltage (AC) even though many devices, particularly those involving electronic circuits utilize power at a substantially constant voltage referred to as direct current (DC) due to the ability to transmit power at high voltage and relatively low current for substantial distance with reduced losses and less costly infrastructure and the ability to easily derive low voltage and higher currents near the location where power is utilized with transformers. Power converters capable of converting AC power to DC power have also become very sophisticated in design for high efficiency, high power density and low cost.
In recent years, however, there has been increasing interest is generating power from so-called renewable resources such as solar and wind power which can often be provided in a geographically distributed manner and located more proximate to the location where power is utilized. However, such sources of energy from which power may be generated may be only intermittently available; thus requiring power storage. Power storage is also of increasing interest in order to balance loads on power distribution grids and distribute more power over presently available infrastructure. Substitution of electrical power for mobile devices, such as vehicles, to which power cannot be connected when in use also requires storage of power.
Storage of power can be achieved in many ways. However, at the present time, it is most practical to store power in the form of charge in batteries which implies storing and recovering power through use of direct current. It is therefore desirable for power converters to be designed to provide for bi-directional operation to transfer power both into and out of a storage device and to provide conversion between AC and DC power. Numerous designs of power converters providing such functions have been developed and improvements in capacity, power density, reliability, size, efficiency and reductions in cost are continually being sought.
At the present time, most sources of inefficiency in power converters is the power consumed during the turn-on and turn-off of switches at high frequency and limitations of design and operating parameters of available semiconductor switches of sufficient current-carrying capacity. For example, power converters capable of providing substantial power usually include insulated gate bipolar transistors (IGBT), integrated gate commutated transistors (IGCTs) and gate turn=off thyristors (GTOs) for high current carrying capability and tolerance for electrical stress (which generally must be provided by protective circuitry and/or converter design) but which exhibit high switching energy consumption and significant diode reverse recovery time which limits the frequency at which they can be operated; often to a frequency as low as nine times the fundamental frequency of the AC waveform input or output while operation at high frequencies is desirable to allow reduction in size of magnetic elements and/or capacitors used in AC input or output filters. The power consumed by switching not only decreases the efficiency of the power converter but must be dissipated as heat; requiring heat sink structures or other cooling arrangements that increase size and weight and reduce power density of power converters.
In general, it is desirable to provide for the switches in power converters to be controlled in such a manner that switching is performed when the voltage across the switches is zero or near-zero, referred to as zero voltage switching (ZVS) or zero voltage transition (ZVT). (The term “soft switching” may also be used but should be understood as a collective reference to numerous techniques, including ZVS and ZVT, of reducing electrical stress and energy consumption during switching.) However, for some power converter topologies, ZVS or soft switching can only be achieved at the cost of generating very complex switch control waveforms and/or using resonant circuits which increase the number of electrical elements in the power converter design and thus are a significant factor in size, weight and cost of a power converter as well as having the potential to increase electrical stress on elements of the power converter.